The capacitor should be large with respect to the grid input capacitance and of course High Q, so usually mica capacitors are used. As mica caps didn't used to be easy to make, smaller values e.g. 100 - 500 pF (mmF) are used.

The RC circuit becones a HIGH PASS filter for the audio signal, so the break point (-3DB) should be somewhere between 50 - 500 Hz.

Back "then", the accepted values typically were 250 pF and 1 or 2 Megs. Today, the prevailing consensus among builders of high performance regens is to use the the highest value resistors and the lowest value capacitors in order to minimize the resonant circuit loses and therefore maximize Q. A common grid leak now is 50 pF and 10 Megs. I have heard of some using 100 M resistors and 10 pF capacitors.

Anything less than 50 pF coupling capacitor would have significantlosses to the loading capacitance. 250 pF sounds better to me..

Lightening the loading to the minimum possible is a worthy goal. It has a notable effect on the Q of the tank. That said, most 20s radios weren't particularly interested in high tank Q because 1) they were BCB only, 2) they were doing a juggling act to provide 'not-too-wide' bandwidth at 1500 kc and 'not-too-narrow' at 550 kc.

For new homebrew regen circuits I usually settle on 40-70pf and several megs. (4.7-5.6 etc). For the old dogs I use whats there already.

I read a history yesterday that said the first radios had no resistor.Some tubes worked and some didn't. Only the ones with a little gas left in them that "leaked" off the charge worked.

Now that we have become much more intelligent and have no "leaky" tubes to use for detectors, we must make a determination of what to use.

Armstrong’s original regenerative circuit did not have a grid leak, but his superhet had them on both detector tubes a few years later. Evidently, they were intelligent enough by WWI not to have "leaky" tubes. But then I am not familiar with the theory you read and would like to learn more about it. The only one I read about claims the grid condensers had leakage, so I really don’t know what to believe. Maybe both of them are true to some extent. However, I do know that at least a few early battery sets with hard vacuum tubes worked just as well with the resistor removed, so they must have had leakage other than the tube.

During the early days, a soft pencil mark was used for a grid leak, and some manufactured sets had the contacts on the front panel for using this method. Most sets were homemade then and the builder had to devise his own method for using a pencil mark. They just experimented with it until they found the best resistance for their rig. They soon found that the least amount they could make work usually gave the best results.

The cheap manufactured grid leaks in a glass tube that finally became available didn’t work as well as that, but they were a lot more convenient to use. They were just a narrow strip of thin cardboard soaked in India ink, so they were not much different than a pencil mark. Since they were not adjustable by the user, all he could hope for was that the value would be somewhere near what was printed on the label. The value on the label was not much better than a guess, and all the factory could hope for is that it was somewhere in the ballpark. Even if it had been accurate, the humidity could change over night and it would be something else the next day.

The tolerance of many mica grid caps can vary all over the place if they are stamped together with eyelets. Some of them will change if a screw is inserted in the hole and tightened down with a nut. I have a good number of NIB Dubilier caps with eylets, both with grid leak clips and without. Of those I have tested, some are reasonably close, but others are off a good bit more than 20%. Fortunately these variables can be hand picked for such things as peaking old IF transformers or whatever.

When an ‘01A tube was used, the grid leak and condenser values were not all that critical for the broadcast band. Most of the time the standard values worked well, even with a wide tolerance. The experienced amateur knew to tweak his set for best performance at the higher frequencies, and was more apt to have a soft detector.

Leigh, Grebe was an amateur from the get go, so I imagine he was more critical about these values. His MU-1 with the switched coils was the only 3 dialer I have owned that would tune above the modern broadcast band without modifying it.

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PS is there a spell checker on this forum?

My spell checker is Microsoft Word and I copy and paste to the forum.

_________________What we do today is the continuing history of radio - br

Of those I have tested, some are reasonably close, but others are off a good bit more than 20%. Fortunately these variables can be hand picked for such things as peaking old IF transformers or whatever.

Leigh, Grebe was an amateur from the get go, so I imagine he was more critical about these values. His MU-1 with the switched coils was the only 3 dialer I have owned that would tune above the modern broadcast band without modifying it.

Hi Billy,

Take a look at the capacitor article on the Articles page of my Synchrophase site for an interesting take on how Grebe adjusted his caps to less than 5% error. Very ingenious idea.

And Grebe had a lot more experience with this stuff than most manufacturers in the early 1920's. He'd already been in the business for over ten years. He published his first catalog in 1914, having started the business ca. 1912.

The tube I did the most experimenting with was the '76. I used DC on the heater. I ended up in the 10-30V range for the plate. When the grid leak cap got under about 25PF I began to hear a buzz in the tin cans. That was with a 10 M resistor.

IF I was going to grab a value that gave the best comprimise it would be about a 5M and 50pf.

I never messed with it but, one could put the grid leak on a tap in the middle of the coil, much like a crystal set. This would reduce loading. I never tried this one wither but one could probably use a pick up coil, like the Mystery crystal set. When you think about it it is basically the same as the low value cap though.

I've been only concerned with the audio cut off, but EXray points out that the major concern was loading of the tuned circuit, making the "Q" different at different parts of the broadcast band.

So, the variable capacitor value is of concern across the band.
I see that adjustable capacitors available today are 365 pF.
Were the more than that on older radios?

Bandwidth required for OTRs might be about 10 KHz. So at the
bottom of the band 550 KHz, Q = 550/10 = 55

If the Tuning capacitor were 365 pF
The reactance Xc = 1/2pifc = 790 ohms
If we wanted a Q = 55, the loading, R = Q x Xc = 55 x 790 = 43,450
Pretty low value if the loading R is all that is contributing to Q.
I wonder what the Q of the tuning coil is? much larger than 55?

Considering the top end, about 3 times the frequency would require 1/9 the capacitance or 9 times the reactance.
790 x 9 x 55 = 390,000 ohms Still small compared to resistor load.
But of course, the Q of the coil itself is probably less than that caused by the resistive loading.

After a few calculations on Q and resistive loading, I can't see how the resistive loading can effect the Q of the tuning circuit unless the resistive value gets below 1 M ohm.

I fail to see how the RC values would have any noticable effect upon the audio bandpass. The purpose of the grid leak, as I understand it, is to shift the operating curve of the tube into a nonlinear region and thereby enable demodulation of the RF signal to take place. The resistor allows the grid to stay a few volts negative due to the space charge and the capacitor feeds the feeble RF signal directly to the grid.

With most of these early 20s sets, taking Q into consideration is pointless. Many manufacturers even relied upon purposely lossy circuits in order to suppress unwanted oscillations without paying the royalties on the neutralization patent. Today, it's not uncommon to see broadcast band coils with a Q of around 1000. A Q of say 400 to 600 is commonplace among builders of high performance crystal sets and regens.

I will add that the value of the grid leak resistor will affect the negative voltage the grid attains. By altering the value of the resistor, the tube's negative bias can be shifted towads a region more conducive to demodulation. IMHO, playing around with grid leak values and noting a slight peak in performance isn't so much due to reduced tank circuit loading but rather finding a bias level that maximizes detection efficiancy.

I've been only concerned with the audio cut off, but EXray points out that the major concern was loading of the tuned circuit, making the "Q" different at different parts of the broadcast band.

I guess I didn't make my point very clearly because thats not quite what I intended to convey.
The old 20s radios, by nature, and I'll single out the more common typical 3-diallers and later ganged one diallers-not experimenter models,
have an inherent and unavoidable difference in tuned circuit Q between say 550 and 1500. As DXer suggests, the Q has to be less than optimum to achieve a good average bandwidth across that three-octave range. Given that the Q is generally stinko in comparison with what COULD be done (then and now) the plain old .00025 mmf is just fine for coupling.. Its relatively difficult to additionally load down a low-Q circuit and the cap and loading play a smaller role in the Q-equation than would be the case otherwise.

In regenerative circuits where the user has some degree of control of selectivity by variable regeneration it ( the cap) does play a role, and in shortwave circuits it clearly does, in that the Q/selectivity CAN be optimized. Hence, its advisable lighten the loading as far as you can.

Grid leak purpose: The purpose of the resistor is to drain the charge off of the grid side of the coupling capacitor. The charge is made by the RF signal that is rectified by the grid's conduction for positive signals.

Effect of RC values: If the values drained off the charge fast enough to reduce the level of 1 KHz audio signal, it wouldn't be amplified. Therefore, these values determine the low frequency break pont of the audio bandpass.

Q consideration: If they purposely lowered Q to stop oscillation, they also reduced gain. If the Q of modern radios is that high, low audio bandwidth must result.

Grid voltage: This will be determined by the RF level of the signal.
The average value will be half way between zero and the peak to peak level of the RF signal. The value of the resistor just determines how fast the charge is "leaked" off.

I wonder how "peak performance" is defined?
I have a glass grid leak resistor in a Gilfillan 10 NEUTRODYNE that I'm presently working on.

Setting the neutralizing capacitors is tricky. It has a 3rd capacitor that I don't know how to adjust.

Just hit the quote button at the top of the post you're replying to. If you want to break it up like I'm doing here you can add in your own

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Grid leak purpose: The purpose of the resistor is to drain the charge off of the grid side of the coupling capacitor. The charge is made by the RF signal that is rectified by the grid's conduction for positive signals.

True, and this gives the user a point at which he can control (somewhat) the amount of leak to the benefit of weak signals. When preceeded by other stages I *think* there's some interstage matching benefit as well by tweaking the value as well as biasing. I don't know all the mechanisms here - I'm just relating how it appears to work in a circuit.

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Effect of RC values: If the values drained off the charge fast enough to reduce the level of 1 KHz audio signal, it wouldn't be amplified. Therefore, these values determine the low frequency break pont of the audio bandpass.

I must admit I'm not really following that concept. I'm thinking in terms of an RF detector and don't see audio freqs at the grid???

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Q consideration: If they purposely lowered Q to stop oscillation, they also reduced gain. If the Q of modern radios is that high, low audio bandwidth must result.

Reduction of gain is a proven, effective way to discourage oscillation! Modern radios are generally superhet where the IF dictates selectivity and the xfmrs are typically designed with that purpose in mind....and no, the typical broadcast radio doesn't utilize very hi-Q in the IF. That said, a good crystal set can easily limit audio response at the low end of the band with a hi-q coil.

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Grid voltage: This will be determined by the RF level of the signal.The average value will be half way between zero and the peak to peak level of the RF signal. The value of the resistor just determines how fast the charge is "leaked" off.

OK. Just shooting from the hip, wouldn't there be an advantage in 'moving' that point when dealing with weaker signals...and maybe higher frequency rf signals as well?

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I wonder how "peak performance" is defined?

For me its by ear! There's a number of things that will improve selectivity while worsening sensitivity so I would submit that "peak performance" is really a different way of saying "most acceptable compromise" in the context of what we are discussing.

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Setting the neutralizing capacitors is tricky. It has a 3rd capacitor that I don't know how to adjust.

There's various techniques but the quickie way is to just make sure it doesn't oscillate. In ham gear we always tweak the neutralization for max output at minimum current. Although I've never approached an old set that way I don't see why you couldn't. Old triode circuits aren't guaranteed to require neutralization.

Grid leak purpose: The purpose of the resistor is to drain the charge off of the grid side of the coupling capacitor. The charge is made by the RF signal that is rectified by the grid's conduction for positive signals.

I disagree. Yes, that is the explanation repeated in many a radio textbook but regardless of how I attempt to approach it, I cannot accept it as being correct. The RF voltage across a tuned circuit on a strong station can reach a volt or two (or 30V in the case of a former nearby flamethrower) but even a medium strength signal will only produce a few millivolts. I cannot believe that those few millivolts will be rectified by the grid and contribute noticeably to the grid bias. Most of the negative bias is due to the space charge present on virtually every thermonic device under static conditions. A normal RF signal is not going to affect that bias.

I have a glass grid leak resistor in a Gilfillan 10 NEUTRODYNE that I'm presently working on. Setting the neutralizing capacitors is tricky. It has a 3rd capacitor that I don't know how to adjust.

Hi John,

I'm not familiar with that particular radio. However, there may be an analog in the Grebe Synchrophase. It has three tuning caps, all identical including a mica trimmer.

On the first two stages, this trimmer is the neutralizing capacitor. But on the third stage (the input to the detector) it sets the tracking of that tuning cap against the middle stage. First stage tracking varies all over the map depending on the antenna.

But on the third stage (the input to the detector) it sets the tracking of that tuning cap against the middle stage. First stage tracking varies all over the map depending on the antenna.

Leigh may actually be right this time, arr... arr.. I don't know your Gilfillan, and can't readily find a schematic, but its rather common to have one section of a 3-ganger with a tracking trimmer. Not always on the last stage, though. One of my one-knob, 3-gang sets...and I forget which one...used a front panel tweak control that actually mechanically moved the frame of one of the individual caps for this tweaking purpose since all three of the rotors were ganged solidly upon initial 'coarse' alignment.

Have found schematics that are close to the Gilfillan 10, but no exact ones. But I have followed out the wiring to know what it is.

Two TRF RF amplifiers, grid leak detector transformer coupled to a second audio amplifier, transformer coupled to output stage. I have added my own output transformer to drive a speaker.

There is a separate input tuning capacitor for the first RF stage.
The input tuning to the second stage and the detector stage are ganged. All of the variable capacitors have a trimming capacity in parallel.

The two RF stages are neutralized with a small adjustable capacitor from the secondary of its plate transformer back to it's grid. Normal expected neutralization. Have these two adjusted well by normal means.

There is a third capacitor from the second RF amp plate back to the low side of the antenna input tuning coil. Don't know how to adjust it. Still have oscillation at full RF gain, with full fillament voltage on.

How did they do it?

Tell me how to get a picture or illustration on the messages & I will show it.